Proportional cycling switch



NOV. 15, 1949 A. SENN PROPORTIONAL CYCLING SWITCH Filed Aug. l, 1945 IshJe A. SEN/v INVENTOR.

Patented Nov. 1s, 1949 UNITED STATES PATENT vOFFICE PROPORTIONAL CYCLINGSWITCH Jurg A. Senn, Washington, D. C.

Application August l, 1945, Serial No. 608,167

12 Claims. (Cl. 200-136.3)

This invention relates to cycling switches adapted to automaticallysupply electric power to using devices at varying time intervals andmore specifically to those in which the length of the On and Oi cyclesdepends on both the actual temperature and the desired temperature.

Gouy in the French publication: Journal de Physique 6, 479 (1897)describes a thermoregulator in which a platinum wire is extendeddownwardly into contact with the mercury meniscus ofV a U-tube which isclosed at one end and thus serves as a temperature indicator. Theplatinum wire is oscillated vertically by an oil-center cam into and outof the mercury to control the energization of the electric heatingcircuit for the temperature indicator. The connection to his heater issuch that energy is periodically supplied to the bath during that partof the cycle of motion of the platinum wire duringwhich the latter isnot in contact with the mercury. lAn increase in the temperature of themercury bulb results in the meniscus rising which causes a decrease inthe length of time during which energy is periodically supplied to thebath, thus tending to stabilize the bath temperature.

One of the most serious, objections to Gouys oi-center cam type ofswitch contact actuator is the variable spacing of the contacts in theiropen position. This is most objectionable when the platinum wire iswithdrawn from the mercury meniscus for such a slight distance that thecircuit across the contacts is not opened but continues as an arc,burning and pitting the oo ntacts. In Gouy this variation in the spacingof the contact surfaces due to burning does not change the adjustmentsince one of the contacts (mercury) is a part of thetemperature-responsive element. In d'isc or button-contact switchesusing 'the Gouy arrangement, however, contact spacing is one of thefactors afiectingtemperature calibration and any reduction of this spacenecessitates recalibration of the device.

Since the eccentricity of the cycling cam should wherever possible dueto the risk of shorting through broken insulation, breakage due toexcessive twisting, etc. The present invention eliminates each of theforegoing disadvantages and in addition provides further advantageswhich will be described.

Therefore, it is an object toprovde a cyclic switch operating on theGouy principle but modied to provide contacts in which the make andbreak travel is uniform over the entire range of operation of theswitch.

Another object is .to provide a switch of the character specified inwhich variations in the thickness of the contacts due to wear, pitting,burning or other cause does not affect the calibration of the switch.

A further object is to provide a cyclic switch of the character speciedin which the range of stabilizing regulation is not dependent on theeccentricity of an off-center cam, but instead is limited only by thedimensions of a cylindrical cam, thereby greatly increasing the On-Oil,or stabilizing range and making the switch more compact.

Another object is to provide a proportional,

cycling switch utilizing the Gouy principle in a superposed,spring-biased lever system which is virtually free of lost motion ordeformation of the parts over a long, switch-operating life.

A further object is to provide a proportional, cycling switch utilizingthe Gouy principle in a multiple lever system that is protected fromexcessive biasing forces by a tensioned, follow-up spring.

A further object is to provide a switch of the character specifiedhaving a multiple lever system in which the biasing action of thetemperature indicating means as well as that of the temperature settingcam is applied close to the fulcrum of the main lever, thereby lesseningthe risk of deformation of the levers and consequent be large ascompared with the movement necesinaccurate temperature responsiveness ofthe switch.

Another object is to provide a proportional, cycling switch utilizing aperipheral, circular, disc-cam as the temperature adjustment meansthereby obtaining virtually 360 degrees of adjustment in stepless,straightline or other pattern of control as desired.

Another object is to provide a switch of the character described that isinexpensive and is easily manufactured, assembled and adjusted.

Further objects and advantages will be apparent as the descriptionproceeds.

Reference is now made to the drawing wherein Fig. 1 is a plan view of apreferred form of cyclic switch embodyinar the principles of myinvention, partly broken away along the line I-I of Fig. 2, the switchcontacts being omitted for clarity and the cycling cam lever shownpartly broken away;

Fig. 2 is a sectional view of my switch taken on the line 2-2 of Fig. 1,the switch contacts and their carrier being shown in elevation and thecycling cam lever of Fig. l being shown in its operative relation to thecycling cam; and

Fig. 3 is an end, sectional View of my improved switch taken on the line3-3 of Fig. 1, the switch contacts and the carrier thereof being shownin elevation.

In the drawing there is shown at I a box-like,

outer housing on one wall of which is mounted by means of screw 2 andnut 3, a circular cam-disc 4 having an inclined periphery providing acam surface 4a. Cam-disc 4 may be rotated to any desired setting bymeans of the combined knob and dial 5. On the opposite wall of the outer-housing I and to one side is mounted the nipple block Ii of theflexible diaphragm assembly and held in place by nut 1 threaded thereon.Block 5 carries the lower wall 8 of the diaphragm as,- sembly while theupper diaphragm wall 9 carries a stud III adapted to transmit pressurefrom the expansion of the diaphragm walls to a ball II as will later bedescribed. The diaphragm walls are peripherally sealed and are adaptedto separate and move stud I Il upwardly upon the ingress of uid throughthe bore in block 6 as will be understood.

An inner housing I2 adapted to nest within the open end portion of outerhousing I is formed with side walls I3 and I4 the latter of which has aninwardly onset, central portion I4a, a top wall I5 and a bottom wall I6having an upwardly-turned portion I6a. Walls I5 and I6 are formed withoppositely disposed depressions I5b and I6b in which is journaled ahinge pin I1. The latter serves as a pivotal mounting for a bell-cranklever I8 having a centrally disposed, lower ear I9 and an opposite,upper ange 20 which extends across almost the full width of the innerhousingl2. One end of a strip 2| of spring steel is united to flange 20by means of rivets 22 and backingplate 23 and its other end is united tothe end of cycling cam lever 24 by rivets 22 and backing plate 26. Itwill be understood that rivets 22 are repeated at intervals in thesuperposed portions of the united members (Fig. 1) to give permanent andsecure connection of these parts.

Cycling cam lever 24 is generally triangular in shape and is stiiened bythe formation of anges 24a in its marginal edge portions. The left endof this lever narrows into a rectangular portion 24h and terminates in acam-engaging portion 25, which like the flanges 24a is bent downwardlyout of the general plane of the cam lever. A cylindrical cam 21 formedby moulding en bloc or by cutting an outer tube 21a to the outline shownin Figs. l and 2 and driving it tightly over a solid cylinder 28 whichis mounted on a synchronously drive shaft 29, engages portion 25 of thecycling cam lever 24 during a half revolution of the cam and raises theleft end of lever 24 through a distance equal to the height of the camwhen the parts are arranged as shown in Fig. 2. A riser block 30 made,preferably of a rigid, insulating material and having a wall 3l ofarcuate shape corresponding to its distance from hinge pin l1 is mountedon the cycling cam lever 24 by means such as rivets 32.

Normally closed switch contacts 33, 34 are mounted on a spring arm 35and a conductor strip 35, respectively, and the assembly suitablyinsulated from and fixed to the wall I5 of the inner housing as byinsulating spacers 31 and any desired means such as screws 38 and others(not shown) which may extend upwardly through insulating spacer blocks39 from wall I5. I prefer to employ a pair of spaced, conductor strips3B provided with terminal connections 40 and contacts 34 and bridgethese contacts by means of a pair of electrically interconnected,similar contacts 33 fixed to the spring arm 35, thus providing two pathsfor the electric current 'oetween terminals 43.

Particular attention is directed to the shape of cam 21. While the innermargin of the tube portion 21a may have any desired shape depending onwhether a straight line or other relationship is desired between the Onand Ol periods, in the preferred form, the tube 21a extends halfwayaround the cylinder 28 at the longitudinal midpoint of the latter, thusgiving equal On and Off intervals at this point. The leading edge 21h ofthe tube 21 is so inclined as to open the contacts 33, 34 during theoptimum time interval, i. e. over at least one-half cycle of the A. C.power supply so that the circuit across the contacts will be broken asthe current passes through zero. The inclination of cam edge 21b alsocorresponds to the slope of portion 25 of the cycling cam lever 24 andis preferably curved about the hinge pin I 1 as a center so that thelifting force of the cam edge 21a is exerted in line with cam lever 24.The trailing edge 21e of the tube 21 is radial to the cam axis toproduce rapid closure of the contacts. It is obvious that thesefunctions of tube 21a and cylinder 28 with respect to the operation ofthe contacts could be interchanged if desired. The position of the leverportion 25 along the longitudinal axis of the synchronously rotated camwill, of course, determine for what part of a cam revolution thecontacts 33, 34 are openedand closed. The factors and mechanism whichdetermine the position of this levei` in its scanning movement acrossthe cam will next be described.

The upwardly turned portion ISa of bottom wall I6 is formed with adepression Ib which serves as a retainer for a coil spring 4I the otherend of which seats in a depression I8a in the bell crank lever I8 tourge the latter counterclockwise (Fig, l) to the position indicated bythe dotted lines |80, and rotate the cycling cam lever 24 to its full Onposition at the lower end of cylinder 28. This normally 0n position oflever 24 is subject to the inuence of two other factors, viz: (1) thesetting of the range dial 5 and (2) the pressure exerted by thediaphragm stud I as will now be described.

A diaphragm lever 42 located centrally of the inner housing I2 and outerhousing I has a downwardly-inclined retaining aperture 43 for ball IIwhich is held in its retained position by a at, follow-up spring 44, theremote end of which is iixed to an intermediate portion of lever 42 bymeans of spacer 45 and rivet 46. A coil spring 41 somewhat weaker thanspring 4I is centered beneath the floating lever 42 by the head of rivet46 and its lower end is retained by a raised portion |3a of side wall I3to bias lever 42 upwardly. The right end of floating lever 42 is offsetdownwardly from the portion just described and has fixed therein ahardened, pointed stud 48 in engagement with a hardened, disc-plate 46fixed in the central portion of a wedge-shaped arm 18d which is normalto the major arm ofbell crank I6 and integral therewith as shown in Fl.1.

lie left end of floating lever 42 is bent downwardly and then sharply upat 50 to provide an angular seat for a pair of knife edges 5I, 52 formedon a pair of downwardly bent, spaced portions oi' a lever extension 53.The latter is xed to a stiff spring strip 54 by means of rivets 55 whichsecure also a superposed, adjustable lever 56 having a slightly offset,left end 51 contacted by an adjustment screw 58 threaded through a boss59 formed in the central portion 66 of lever extension 53 and accessiblethrough an aperture (not shown) provided in the closed end wall ofhousing 1. The bent, spaced portions of lever extension 53 have marginalstops 6l which serve as guides for the floating lever 42. Rivets 62 andbacking plate 63 serve to unite the strip 54 to the offset portion |4a.

It will be seen that lever 56 and its extension 53 provide a fulcrumconstituted by the knife edges 5I, 52 and adjustable vertically (Fig. 1)upon rotation of cam disc 4 and also easily calibrated to its zerosetting by means of adjustment screw 5 8. Floating lever 42 is thusbiased upwardly by spring 41 so that stud 48 always contacts disc plate49, but since spring 41 is weaker than spring 4I, the latter will rotatecycling cam lever 24 clockwise to its full On position with respect tocycling cam 21 until the remotely located, thermal feeler orpressure-responsive member (not shown) causes the diaphragm 8 to expand,moving the stud ID and lever 42 upwardly, thereby slightly rotatingcycling cam lever 24 in a clockwise direction toward the Off position.

The operation of my switch for electric range use, for example, is asfollows: Assume that terminals 40 are connected in series with the`power supply to energize an annular range heating unit and that nippleblock 6 of the flexible diaphragm 8 is connected by a suitable conduitwith a thermal bulb or feeler member located centrally of the heatingunit as is shown, for example, by the Weber et al. Patent No. 2,303,012.The knob 5 is turned to the desired temperature setting for theparticular cooking operation from its Oil? position wherein the highestand preferably stepped portion of cam 4a presses downwardly on lever 51to maintain cycling Acam lever 24 in its Off position at the upper endof cam 21. Rotation of cam 4a raises fulcrum knife edges 5l, 52 so thatspring 4l acting through bell crank I8 moves the cycling cam lever tofull On position and moves the right end of lever 42 downwardly aboutthe edges 5|, 52 as a fulcrum since nipple block 6 in the cold positionof diaphragm 6, 9 is ineffective as a fulcrum. As the heating unitraises the temperature of the cooking vessel to the setting of dial 5,the upward movement of diaphram 6, 9 and of stud l0 raises the ball l land through it, the follow-up spring 44 and the right end of lever 42 torotate the cycling cam lever 24 clockwise to an intermediate position onthe cam 26 in which the heat supplied just equals the heat losses of theheated system.

In addition to its function of transmitting movement to the lever 42,follow-up spring 44 has the right end thereof upwardly (Fig. 1) and itsfollower portion 25 against the straight, radial wall 21e of cam 21. Thetop of ball Il will flex the follow-up spring 44 upwardly suiiiclentlyto absorb the difference in height of cam 4a that was not utilized inmoving follower portion 25 to the radial wall. This condition willcontinue until the rotation of cam 26 has brought the inclined wall 21aagainst the follower portion 25 raising the same so that it may snap tothe fully Oil!v portion of cam 21. Overshooting is prevented byengagement of bell crank I8 with the casing I6 as shown by the dottedline 64; similar engagement is indicated by dotted line I8c preventsovershooting of the cyclical cam lever 24 in the fully On position.

It will be apparent, of course, that portion 25 could be provided with alaterally'upturned lip so that it would immediately surmount the radialwall 21o of cam 21. 'I'hls arrangement would be inferior to that shown,however, since it would not provide the desired quick closure ofcontacts 33, 34 as portion 25 drops down over the radial cam wall at 21e(Fig. 1).

In addition to the greatly amplified movement of diaphragm 8, 9 and cam4a provided by my lever arrangement and resulting in the relativelylarge, stabilizing excursion of follower 25 axially of cyclic cam 28with ,consequent temperature control stability, other important featuresof my novel switch will be apparent. Cam 4 and adjustment screw 58 arein vertical alignment and only slightly offset from the knife edges 5|,52 so that bending stresses in levers 53 and 56 are minimized. Exceptfor the stepped Off portion of cam 4a the entire 360 degree sweep ofthis surface may be utilized to spread the temperature range over alarge, angular movement. Also the cam inclination need not be constantbut may be steep through a narrow angle from the Oif position to a lowtemperature, say 200 degrees F. permitting more than A1 of the camlength to be utilized for the temperature range most used, from 200 to500 degrees F. for example. The use of knife-edges and spring-biasing inthe lever system eliminates lost motion and assures continued accuracyof calibration. While cam 26 is rotated at a-uniform and preferably,slow speed, say one revolution per minute, as long as any of the fourswitches grouped thereabout are in On position (indicated by the radialarrows in Fig. 2) the form of switching means for the driving motor ofthis cam is not a part of this invention since various arrangements forthis purpose will be obvious to those skilled in the art.

It will be apparent that the preferred form of my invention comprises'alaterally movable cam lever having a follower portion which sweepsaxially across a generally cylindrica1, rotary switch actuating member,the lateral position of the former being dependent` upon the indicatedand desired temperatures of the heated body, and the surface of themember being divided into axially variable portions, certain of whichtogether with the follower portion effect closure of the switch contactsand other portions of which similarly effect opening of said contacts.However,

agencias other forms of my invention will be readily apparent.

Hence, instead of utilizing the cylindrical, stepped cam as theactuating means for tie cycling cam lever 24, a rotary membercorresponding to the hollow cam 2l' could be used to raise the lever 24and a transverse stop mounted on the left wall of the housing I (Fig. 2)could serve to limit the lower movement of said lever. Since theforegoing as well as other variations of my preferred form will beobvious to those skilled in the art, they have not been illustrated nordescribed in detail. However, I do not wish to be limited to the preciseconstruction specifically disclosed as the preferred embodiment of myinvention but only as may be fairly within the scope of the followingclaims:

1. A switch comprising normally closed electric contacts, a rotary camhaving an outer cylindrical surface terminating in an axially inclinedmarginal wall substantially coextensive circumferentially with saidsurface and also normal thereto and the extremities of which are joinedby a wall which is inclined to said surface and generally parallel tothe axis thereof, a follower yieldably mounted to alternately engagesaid walls during the rotation of said cam and to be moved thereby intoopening and closing relationship, respectively with said contacts andalso pivotally mounted on an axis which is normal to the axis of saidcam, pressure responsive means disposed beneath said follower andbetween said cam and pivotal mounting for moving said follower arcuatelyacross said cam in a direction generally parallel to the axis of thelatter whereby the duration of the open and closed conditions of saidcontacts will be a function of the pressure exerted by said means, andsimilarly disposed stop means spaced from said pressure responsive meansfor limiting the effectiveness thereof.

2. A switch comprising electric contacts, a rotary cam having an outercylindrical surface terminating in an axially inclined marginal wallsubstantially coextensive circumferentially with said surface and theextremities of which are joined by an arcuate wall which is generallyparallel to theaxis of said surface, a follower yieldably mounted at itsremote end to urge its opposite end into engagement with said wallsalternately during the rotation of said cam and to be moved thereby intoopening and closing relationship, respectively with said contacts, saidfollower being also pivotally mounted at the center of curvature of saidarcuate wall, pressure responsive means disposed between said pivotalmounting and said cam for moving the opposite end of said follower aboutits pivotal mounting and across said cam in a direction generallyparallel to the axis of the latter whereby the duration of the open andclosed conditions of said contacts will be a function of the pressureexerted by said means.

3. The combination set forth in claim 2 wherein said marginal wall isnormal to said cylindrical surface, said arcuate wall is inclinedthereto, and the cam engaging end of said follower is provided wth apair of surfaces having the same inclination as said walls insuringquick closing and opening, respectively of said contacts.

4. A switch comprising electrical contacts, a rotary cam having acircumferentially continuous, outer, cylindrical surface terminating inan axially inclined marginal Wall substantially coextensivecircumferentially with said surface and the extremities of which arejoined by an arcuate wall which is generally parallel to the axis ofsaid surface, a casing disposed to one side of said cam and includingparallel, top and bottom walls and a connecting end wall, contactcarrying means carried by said top wall, a vertically disposed hingeextending through said parallel walls at the center of curvature of saidarcuate wall, a follower yieldably mounted at its remote end to urge itsopposite end into engagement with the walls of said cam alternatelyduring the rotation of the latter and to be moved thereby into openingand closing relationship, respectively with said contacts, said followerbeing also pivotally mounted on said hinge, pressure responsive meansdisposed between said parallel walls for moving the opposite end of saidfollower aboutits pivotal mounting and across said cam in a directiongenerally parallel to the axis of the latter whereby the duration of theopen and closed conditions of said contacts will be a function of thepressure exerted by said means.

5. A switch for electric heaters comprising electric contacts havingopen and closed positions, an elongated, pivotally mounted cycling leverincluding a cam follower portion and a transversely disposed, contactactuating portion, a cylindrical cam rotatable about an axis normal tothe pivotal axis of said lever and having an axially variable, elevatedportion corresponding to one of the positions of said contacts, and aninclined, uniformly stepped portion generally curved about the axis ofsaid lever and adjacent said elevated portion for lifting said followerto the latter portion of said cam to produce uniform relative movementof said contacts, means including a relatively strong compression springbiasing said lever about its pivotal mounting in one direction, meansincluding a relatively weak compression spring biasing said lever intheopposite direction, means responsive to the temperature of said heaterfor reinforcing said latter spring to move said lever in said latterdirection, and a manually adjustable stop actingr in opposition to saidlast-named means for limiting the movement of said lever in the latterdirection whereby the open and closed intervals, respectively of saidcontacts will stabilize the energy supplied to said heater at thetemperature corresponding to the adjusted position of said stop.

6. A switch for electric heaters comprising electric contacts havingopen and closed positions, a pivotally mounted cycling lever including acam follower portion and a transversely disposed, contact actuatingportion, a cylindrical cam rotatable about an axis normal to the pivotalaxis of said lever and having an axially variable, elevated portioncorresponding to one of the positions of said contacts and an inclined,uniformly stepped portion generally curved about the pivotal mounting ofsaid lever and adjacent said elevated portion for lifting said followerto the latter portion of said cam to produce uniform relative movementof said contacts, means including a relatively strong compression springbiasing said lever about its axis in one direction, an adjustable leverproviding a .shiftable fulcrum adjacent one end thereof, a floatinglever having one end fulcrumed thereon and the opposite end in forcetransmitting relation to said cycling lever, means including arelatively weak compression spring biasing said oating and cyclinglevers in opposition to said rst-mentioned compression spring, meansresponsive to the temperature of said heater for reinforcing saidrelatively weak spring to move said last mentioned levers against theforce of said relatively strong spring, and a manually adjustable stopadaptedto engage said adjustable lever to determine the position of suchfulcrum.

'1. A switch for electric heaters comprising electric contacts havingopen and closed positions, a pivotally mounted cycling lever including acam follower portion and a transversely disposed, contact-actuatingportion, a cylindrical cam rotatable about an axis normal to the pivotalaxis of said lever and having an axially variable, elevated portioncorresponding to one of the positions of said contacts and an inclined,uniformly stepped portion curved about the pivotal mounting of saidlever and adjacent said elevated portion for lifting said follower tothe latter portion of said cam to produce uniform relative movement ofsaid contacts, means including a relatively strong compression springbiasing said lever about its axis in one direction, an adjustable leverproviding a shiftable fulcrum adjacent one end thereof, an aperturedfloating lever having one end fulcrumed thereon and the opposite end inforce transmitting relation to said cycling lever, means including arelatively weak compression spring biasing said floating and cyclinglevers in opposition to said first-mentioned compression spring, aspring-pressed ball seated in said aperture, means responsive to thetemperature of said heater and adapted to engage said ball forreinforcing said relatively weak spring to move said last mentionedlevers against the force of said relatively strong spring, and amanually adjustable stop adapted to engage said adjustable lever todetermine the position of said fulcrum.

8. A switch comprising normally closed electrical contacts, a rotary camhaving an outer cylindrical surface terminating in an axially inclinedmarginal wall substantially co-extensive circumferentially with saidsurface and the extremities of which are joined by a wall which isgenerally parallel to the axis of said surface, a bell crank leverpivotally mounted about an axis normal to the axis of said cam andhaving an elongated flange extending parallel to the axis of said camand an arm below said flange and extending toward said cam, anelongated, resilient strip carried by said flange, a rigid, generallytriangular cam follower having a base co-extensive with said strip andunited thereto and a narrow apical portion adapted to alernately engagesaid walls during the rotation of said cam and to be moved thereby intoopening and closing relationship, respectively with said contacts,pressure responsive means disposed beneath said cam follower for movingthe latter arcuately across said cam in a direction generally parallelto the axis of the latter whereby the duration of the open and closedconditions of said contacts will be a function of the pressure exertedby said means.

9. A switch comprising electric contacts having open and closedpositions, a-rotary cam having an inner cylindrical surface, a radiallyspaced outer cylindrical surface, an axially inclined marginal wallsubstantially co-extensive circumferentially with said latter surfaceand having terminal portions spaced longitudinally thereof, and a curvedmarginal wall uniting said portions, said Walls being disposed betweensaid surfaces and serving to define said latter surface, a follower forchanging the relative positions of said contacts, said follower beingpivotally mounted at the center of curvature of said latter marginalwall about an axis which is normal to the axis of said surfaces and alsoyieldable radially of said surfaces to alternately engage the latter andto be moved from said inner surface to said outer surface by engagementwith said curved marginal wall, and pressure responsive means for movingsaid follower arcuately across said cam about said pivotal mountingwhereby the duration of the open and closed conditions of said contactswill be a function of the pressure exerted by said means.

10. A switch comprising electric contacts having open and closedpositions, a rotary cam having an inner, cylindrical surface, a radiallyspaced, outer cylindrical surface, a radially disposed, axiallyinclined, marginal wall substantially co-extensive circumferentiallywith said latter surface and having terminal portions spacedlongitudinally thereof and a curved, nonradial marginal wall unitingsaid portions, said walls being disposed between said surfaces andserving tov deilne said latter surface, a follower for changing therelative positions of said contacts, said follower being pivotallymounted at the center of curvature of said latter marginal wall about anaxis which is normal to the axis of said surfaces and also yieldableradially of said surfaces to alternately engage the latter and to bemoved from said inner cylindrical surface to said outer cylindricalsurface by engagement with said curved, marginal wall, /and pressureresponsive means for moving said follower arcuately across said camabout said pivotal mounting whereby the duration of the open and closedconditions of said contacts will be a function of the pressure exertedby said means.

11. A switch comprising normally closed electric contacts, an elongatedarm carrying one of said contacts and having an overhanging portion, arotary cam having an inner cylindrical surface, a radially spaced outercylindrical surface, an axially inclined marginal wall substantiallyco-extensive circumferentially with said latter surface and havingterminal portions spaced longitudinally thereof, and a curved marginalwall uniting said portions, said walls being disposed between saidsurfaces and serving to define said latter surface, a follower pivotallymounted at the center of curvature of said latter marginal wall about anaxis which is normal to the axis of said surfaces and also yieldableradially of said surfaces to alternately engage the latter and to bemoved from said inner surface to said outer surface by engagement -withsaid curved marginal wall, means integral with said follower for movingsaid elongated arm at any pivotal position of said follower and pressureresponsive means for moving said follower arcuately across said camabout said pivotal mounting whereby the duration of the open and closedconditions of said contacts will be a. function of the pressure exertedby said means'.

12. A switch comprising electric contacts having open and closedpositions, a rotary-cam having an inner, cylindrical surface, a radiallyspaced, outer cylindrical surface, a radially disposed, axiallyinclined, marginal wall substantially co-extensive circumferentiallylwith said latter surface and having terminal portions spacedlongitudinally thereof and a curved, nonradial marginal wall unitingsaid portions, said walls being disposed between said surfaces andserving to define saidlatter surface, a follower for changing therelative positions of said contacts and having a cam engaging portion ofthe Same non-radial inclination as said curved, marl ginal wall, saidfollower being pivotally mounted at the center of curvature of saidlatter marginal wall about an axis which is normal to the axis of saidsurfaces and also yieldable radially of said surfaces to alternatelyengage the latter and'to be moved from said inner 'cylindrical surfaceto said outer cylindrical surface by engagement with said curved,marginal wall, and pressure responsive means for moving said followerarcuately across said cam about said pivotal mounting whereby theduration of the open and closed conditions of said contacts will be afunction oi the pressure exerted by said means.

JURG A. SENN.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PAWS Number Name Date 1,812,897- Owens July 7, 19311,962,999 Owens June 12, 1934 2,091,734 Himes Aug. 31, 1937 FOREIGNPA'I'ENTS Number Country Date 353,550 Great I rtain July 30, 1931

